Vacuum tube with substantially constant interelectrode capacitance



March 25, 1941.. R. 1.. FREEMAN VACUUM TUBE WITH SUBSTANTIALLY CONSTANT INTERELIECTRODE CAPACITANCE Filed 001;. 14, 193 9 Dieleciric Material wiin Negafim Goefficien? of Permifliviiy.

Dielectric Material wlth Negative Coefficient of Permittivity.

INVENTOR RBERT L. FREEMAN ATTORNEY atented Mar. 25, 1941 PATENT OFFICE VACUUM TUBE WITH SUBSTANTIALLY CON- STANT INTERELECTRODE CAPACITANCE Robert L. Freeman, Flushing, N. Y., asslgnor to Hazeltine Corporation, a corporation of Delaware Application October 14, 1939, Serial No. 299,430

6 Claims.

This invention relates generally to vacuum tubes and particularly to vacuum tubes designed for operation in ultra-high-frequency modulated-carrier signal-translating systems. A tube incorporating the principles of the invention is of particular utility for use in the circuit of the local oscillator of an ultrahigh-frequency receiver of the superheterodyne type.

In some vacuum-tube signal-translating systems, the interelectrode capacitance of the vacuum tubes utilized is appreciable and must be taken into account in the circuit design. This is particularly true in systems for operation at ultra-high frequencies for the reason that in such systems the interelectrode capacitance of the vacuum tubes utilized is a very appreciable portion of the total circuit capacitance. The interelectrode capacitances of all conventional tubes are influenced to some extent by temperature and such capacitance changes occurring during the warm-up period of a particular systerm may be very disadvantageous. A concrete example may be taken as the local oscillator tube in a television receiver of the superheterodyne type. The oscillator for such a receiver generally operates at such a high frequency that the interelectrode capacitance is an appreciable part of the total capacitance of the frequency-determining circuit of the oscillator. Thus, as such a receiver warms up after being placed in operation, the interelectrode capacitance of the oscillator tube maywhange to such an extent as to vary the oscillator frequency by as much as 200 lcilocycles. Obviously, such a change in the 10- cal oscillator frequency may seriously impair the quality of reproduction, particularly in the sound-signal channel. These interelectrode capacitance changes of the local oscillator tube are probably due principally to (1) variations in geometrical capacitance occasioned by thermal expansion of tube elements and of the envelope, if a conductive envelope is utilized, and (2) changes in the dielectric constant of the various insulating materials utilized in the tubes construction.

It is an object of the invention, therefore, to provide an improved vacuum tube not subject to the above-mentioned disadvantage of vacuum tubes of the prior art.

It is another object of the invention to provide an improved vacuum tube in which the interelectrode capacitances remain substantially constant irrespective of variations in the operating temperature of the tube.

In accordance with the invention, a vacuum tube comprises an envelope, a plurality of electrodes mounted therein, lead-in wires for the electrodes extending through the tube envelope, certain of the electrodes having therebetween undesired capacitance which tends to vary positively with temperature during normal operation, and dielectric material efiectively having a negative temperature coeflicient of permittivity separating the lead-in wires of said certain electrodes and disposed to be heated by the other 10 elements of the vacuum tube during normal operation thereof, the configuration of the dielectric material and its temperature coefiicient oi permittivity being relatively so'proportioned that variations of said undesired capacitance are 15 compensated by variations in the permittivity of the dielectric material.

In a preferred embodiment of the invention the envelope is of metal and the configuration of the dielectric material and the area thereof in contact with the envelope of the tube is so proportioned that changes in the interelectrode capacitances due to heating of the dielectric substantially compensate for undesirable changes in interelectrode capacitances between at least certain of the electrodes of the tube.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Each of Figs. 1 and 2 of the drawing is a perspective view of a vacuum tube incorporating the invention with a portion of the envelope broken 30 away.

Referring now more particularly to Fig. 1 of the drawing, there is shown a vacuum tube in corporating the invention which comprises a plurality of electrodes including a cathode- ID, a control electrode II, and an anode l2, all of the electrodes being enclosed within an envelope l3. A suitable heater I4 is provided for cathode l0. Lead-in wires l6 for electrodes Ill, I I, and I2 and heater M are brought out of the envelope l3 through a button l5 which may be of glass or other suitable insulating material. The envelope l3 may be of metal and the button 15 sealed therewith by means of a metal sealing flange I1. Suitable longitudinal supports l8 are also embedded in button l5 for supporting the various electrodes of the tube, the supports l8 being rigidly held transversely of the tube by an insulating disc l9, preferably of mica, through which they extend. Suitable contact prongs 2B for the i be seen that the envelope i3, button 95, and sealing flange ii are effective to enclose the electrode structure of the tube within a sealed evacuated two temperature coefiicient of permittivity. In accordance with the present invention, therechamber, in accordance with conventional vacuum-tube practice. Conventional vacuum tubes,

" however, as brought out in the preliminary part of this specification, and especially those vacuum tubes which are designed for operation in ultrahigh-frequency circuits, have between certain of the electrodes appreciable undesired interelectrode capacitances which tend to vary positively with temperature during normal operation of the tube. Such variations of interelectrode capacitances, which are sometimes detrimental in the operation of the circuit in which the tubes are used, may be due to the thermal expansion of met- 9.1 parts of the tube or may be caused by heating of the various dielectric elements used in the tum,

such as button l5, which generally have a. posifore, the base 2!, which supports contact prongs 20 and effectively separates the lead-in wires of the tube electrodes, is constructed of a dielectric material having a negative temperature coefficient of permittivity; that is, a dielectric constant which decreases with increase in temperature of the base 2!. .The configuration of dielectric material or base M, the area thereof in contact with the envelope i3, and the temperature coeificient of permittivity of the material are relatively so proportioned that the capacitances between contact prongs 20 through dielectric base Zl decrease, due to heat transferred from metal envelope l3 and from other elements of the tube to base 2! during normal operation of the tube, at substantially the same rate as the interelectrode capacitances between the electrodes of the tube increase. Thus, the variations of undesired interelectrode capacitances are compensated by variation in the permittivity of the dielectric material. Since the total or effective interelectrode capacitances of the tube as connected in a circuit are the sums of the capacitances between the electrodes themselves, their lead-in wires, and the contact prongs, the structure of Fig. 1 is effective to provide a tube, the effective interelectrode capacitances of which are substantially constant irrespective of variations of temperature during operation of the tube.

While applicant does not intend to be limited to any particular dielectric material for use in the insulating-base 2|, the ceramic insulating material known as Alsimag 190 of the American Lava Corporation, Chattanooga, Tennessee, has particularly desirable characteristics. This material comprises 95 per cent titanium dioxide in the form of rutile crystals developed by firing amorphous titanium dioxide, the remaining 5 per cent beinga' glassy or partly crystalline material including aluminum oxide (A1203), silicon dioxide (SiOz) and magnesium oxide (MgO) and has a temperature coeflicient of permittivity between 20 and 80 degrees C of 6.5 l microfarads per microfarad per degree C. Polystyrene resin, which has a temperature coeflicient of permittivity of 0.35 10- micro-microfarads per micro-micro-farad per degree 0., may also be used. A polystyrene resin having such properties is produced by the Dielectric Products Corporation, Jersey City, N. J under the trade name of Victron. This material is a synthetic resin produced by the isolation and subsequent polymerization of the monomeric styrene, CsH-CH=CH2 and is described in Electrochemical Society Reprint 'Zd-SS, page 545. Commercially available Halowax" also exhibits a. negative temperature coeficient of permittivity at ordinary working temperatures of a vacuum tube. Such material might-be used to provide a dielectric base, similar to base 2E described above, even though a contact-supporting structure of a ceramic material were also utilized.

While a triode-has been illustrated in mg. l for the Sake of simplicity, it wili be understood that applicants invention is applicable to vacuum tubes of other electrode structures.

In Fig. 2, there is shown a modified embodiment of the invention which is generally similar to that of Fig. 1 and in which similar circuit elements have identical reference numerals. The tube of Fig. 2 differs from that of Fig. 1 in that it has an envelope 23 of dielectric material and comprises a button or press 24 within which the lead-in wires or contact prongs 20 are sealed. At least a portion of the envelope 23 adjacent the contact prongs 20 is constructed of a dielectric material separating the contact prongs or lead-in wires and disposed to be heated by the other elements of the tube during normal operation thereof. This dielectric material has a negative temperature coefficient of permittivity and its configuration and coeificient of permittivity are relatively so proportioned as to provide capacitance compensation similar to that provided by the base 2! of the tube of Fig. I. For instance, the envelope 23 may be of glass comprising as an ingredient an appreciable amount of titanium dioxide in order to impart thereto a negative temperature coefiicient of permittivity.

It will be understood that the heating of the base 2i of the tube of Fig. 1 and of the press 24 of the tube of Fig. 2 is, to some extent, dependent upon the transfer of heat from contact prongs 20 to the dielectric material therebetween.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A vacuum tube comprising, an envelope, a plurality of electrodes mounted therein, lead-in wires for said electrodes extending through said envelope, certain of said electrodes having therebetween undesired capacitance which tends to vary positively with temperature during normal operation, and dielectric material efiectively having a negative temperature coefficient of permittivity separating the lead-in wires ofsaid certain electrodes and disposed to be heated by the other elements of said vacuum tube during normal operation thereof the configuration of said dielectric material and its temperature coeificient of permittivity being relatively so proportioned that variations of said undesired capacitance are compensated by variations in the permittivity of said dielectric material.

2. A vacuum tube comprising, an envelope, a plurality of electrodes mounted therein, lead-in wires for said electrodes extending through said envelope, certain of said electrodes having there between undesired capacitance which tends to vary positively with temperature during normal operation, and dielectric material comprising an amount of titanium dioxide sufliclent to impart thereto a substantial negative temperature coemcient of permittivity separating the lead-in Wires of said certain electrodes and disposed to be heated by the other elements of said vacuum tube during normal operation thereof the corn iiguration of said dielectric material and its teperature coemcient of permittivity being rela tively so proportioned that variations of said on desired capacitance are compensated by variations in the permittivity of said dielectric material.

3. ll vacuum tube comprising, an envelope, a plurality of electrodes mounted therein, lead -ln vvires for said electrodes extending through said envelope, contact prongs connected to said leadin Wires, certain of said electrodes having therebetvveen undesired capacitance which tends to vary positively with temperature during normal operation, and a base of dielectric material sepa rate irorn said envelope but in intimate contact therewith. and having a negatlvetemperature coedcicient or permittivity, said base supporting said contact prongs, the configuration all said dielectric material and its temperature coecient of permittivity being relatively so prortloned that variations of said undesired capacitances are compensated by variations in the permittivity oi said dielectric material between the. ones oi said prongs which are connected to said certain electrodes.

d. d vacuum tube comprising, an envelope, a plurality oi electrodes mounted therein, and leadin vvires for said electrodes extending through said envelope, certain of said electrodes having therebetvveen undesired capacitance which tends to vary positively with temperature during normal operation, said envelope comprising a por tion adjacent said lead-in wires having a negative temperature coeiilcient oi permittivity dis" posed to be heated by the other elements of said vacu tube during normal operation thereoi, the configuration oi said envelope portion and said negative temperature coeficient of permittivlty being relatively so proportioned that variations of said undesired capacitance are compensated by variations in the permittivity of said dielectric material.

it. h vacuum tube comprising, a metal envelope, a plurality of electrodes mounted therein, lead-in wires for said electrodes, contact prongs connected to said lead-in wires, and a base of dielectric material separate from said envelope and having a negative temperature coemcient oi" permittivity, said base being in intimate contact with said envelope and supporting said contact prongs, the capacltances between certain oi" said electrodes being subject to undesired increases due to heating during operation of said tube, and the con figuration at said dielectric material and the area thereof in contact with said envelope being so proportioned that the intercontact capacitances through said dielectric material decrease due to heat translerred from said envelope to said base during operation oi said tube at substantially the same rate as said undesired capacitance increases,

d. .a. vacuum tube comprising, an envelope, a plurality oi electrodes mounted therein, lead-in vvlres for said electrodes errtending through said envelope, said envelope comprising a portion oi dielectric material separating said leaddn Wires and disposed to be heated by the other elements oi said vacuum tube during normal operation thereof, the capacitances between certain oi said electrodes being subject to undesired increases due'tc heating during operation of said tube, and at least said portion of said dielectric material between said certain electrodes effectively having a negative temperature coeifdcient oil permittivity proportioned substantially to compensate said undesired capacitance increases.

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